U.S. patent application number 09/821701 was filed with the patent office on 2005-12-22 for multiple channel optical frequency mixers for all-optical signal processing.
Invention is credited to Chou, Ming-Hsien, Fejer, Martin M., Parameswaran, Krishnan.
Application Number | 20050280886 09/821701 |
Document ID | / |
Family ID | 35480264 |
Filed Date | 2005-12-22 |
United States Patent
Application |
20050280886 |
Kind Code |
A1 |
Chou, Ming-Hsien ; et
al. |
December 22, 2005 |
MULTIPLE CHANNEL OPTICAL FREQUENCY MIXERS FOR ALL-OPTICAL SIGNAL
PROCESSING
Abstract
A multi-channel optical frequency mixer for all-optical signal
processing and a method for engineering the same. The multi-channel
mixer uses a nonlinear optical material exhibiting an effective
nonlinearity d.sub.eff whose spatial distribution is defined by a
quasi-phase-matching grating, e.g., a QPM grating. The spatial
distribution is defined such that its Fourier transform to the
spatial frequency domain defines at least two wavelength channels
which are quasi-phase-matched for performing optical frequency
mixing. The wavelength channels correspond to dominant Fourier
components and the Fourier transform is appropriately adjusted
using grating parameters such as grating periods, phase reversal
sequences and duty cycles to include an odd or even number of
dominant Fourier components. The multi-channel mixer can perform
frequency mixing operations such as second harmonic generation
(SHG), difference frequency generation (DFG), sum frequency
generation (SFG), and parametric amplification.
Inventors: |
Chou, Ming-Hsien; (Holmdel,
NJ) ; Parameswaran, Krishnan; (Mountain View, CA)
; Fejer, Martin M.; (Menlo Park, CA) |
Correspondence
Address: |
LUMEN INTELLECTUAL PROPERTY SERVICES, INC.
2345 YALE STREET, 2ND FLOOR
PALO ALTO
CA
94306
US
|
Family ID: |
35480264 |
Appl. No.: |
09/821701 |
Filed: |
March 28, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60206874 |
May 24, 2000 |
|
|
|
60266383 |
Feb 1, 2001 |
|
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Current U.S.
Class: |
359/326 |
Current CPC
Class: |
G02F 1/365 20130101;
G02F 1/3544 20130101 |
Class at
Publication: |
359/326 |
International
Class: |
G02F 001/35 |
Claims
1. A multi-channel optical frequency mixer for all-optical signal
processing comprising: a) a nonlinear optical material having an
effective nonlinearity d.sub.eff; b) a quasi-phase-matching grating
defining a spatial distribution of said effective nonlinearity
d.sub.eff in said nonlinear optical material, such that a Fourier
transform of said spatial distribution to the spatial frequency
domain defines at least two short wavelength channels
quasi-phase-matched for performing optical frequency mixing.
2. The multi-channel optical frequency mixer of claim 1, wherein
said Fourier transform of said spatial distribution comprises at
least two dominant Fourier components corresponding to said at
least two short wavelength channels.
3. The multi-channel optical frequency mixer of claim 2, wherein
said Fourier transform of said spatial distribution comprises an
even number of said dominant Fourier components.
4. The multi-channel optical frequency mixer of claim 2, wherein
said Fourier transform of said spatial distribution comprises an
odd number of said dominant Fourier components.
5. The multi-channel optical frequency mixer of claim 2, wherein
said quasi-phase-matching grating has predetermined grating
parameters for producing said at least two dominant Fourier
components.
6. The multi-channel optical frequency mixer of claim 5, wherein
said predetermined grating parameters are selected from the group
consisting of grating periods, phase reversal sequences and duty
cycles.
7. The multi-channel optical frequency mixer of claim 5, wherein
said grating has a uniform grating period superposed by a phase
reversal sequence.
8. The multi-channel optical frequency mixer of claim 7, wherein
said phase reversal sequence has a predetermined duty cycle.
9. The multi-channel optical frequency mixer of claim 2, wherein
said quasi-phase-matching grating further comprises a chirp.
10. The multi-channel optical frequency mixer of claim 2, further
comprising optical structures for in-coupling and out-coupling
light into and out of said quasi-phase-matching grating.
11. The multi-channel optical frequency mixer of claim 1, further
comprising at least one waveguide.
12. The multi-channel optical frequency mixer of claim 11, wherein
said quasi-phase-matching grating is distributed within said at
least one waveguide.
13. The multi-channel optical frequency mixer of claim 11, further
comprising a mode controlling structure.
14. The multi-channel optical frequency mixer of claim 1, wherein
said nonlinear optical material comprises a substrate having at
least one component selected from the group consisting of lithium
niobate, lithium tantalate, MgO:LiNbO.sub.3, Zn:LiNbO.sub.3,
MgO:LiTaO.sub.3, stoichiometric lithium niobate, stoichiometric
lithium tantalate, potassium niobate, KTP, KTA, RTA, RTP and
members of the III-V semiconductor family.
15. The multi-channel optical frequency mixer of claim 14, further
comprising a waveguide in or on said substrate.
16. The multi-channel optical frequency mixer of claim 15, wherein
said waveguide is an in-diffused waveguide.
17. The multi-channel optical frequency mixer of claim 1, further
comprising a polarization control system for rendering said
multi-channel optical frequency mixer polarization diverse.
18. The multi-channel optical frequency mixer of claim 17, wherein
said polarization control system comprises at least one element
selected from the group consisting of polarization mode separator,
polarization rotator, optical isolator, optical circulator, optical
fiber, polarization maintaining fiber and polarization
controller.
19. A method of all-optical signal processing using multi-channel
optical frequency mixing comprising: a) providing a nonlinear
optical material having an effective nonlinearity d.sub.eff; b)
defining a spatial distribution of said effective nonlinearity
d.sub.eff in said nonlinear optical material with a
quasi-phase-matching grating, such that a Fourier transform of said
spatial distribution to the spatial frequency domain defines at
least two short wavelength channels quasi-phase-matched for
performing optical frequency mixing.
20. The method of claim 19, wherein said Fourier transform of said
spatial distribution is defined to have at least two dominant
Fourier components corresponding to said at least two short
wavelength channels.
21. The-method of claim 20, wherein said Fourier transform of said
spatial distribution is defined to have an even number of said
dominant Fourier components.
22. The method of claim 20, wherein said Fourier transform of said
spatial distribution is defined to have an odd number of said
dominant Fourier components.
23. The method of claim 20, wherein said quasi-phase-matching
grating has predetermined grating parameters, and said method
further comprises setting said predetermined grating parameters to
produce said at least two dominant Fourier components.
24. The method of claim 23, wherein said predetermined grating
parameters are selected from the group consisting of grating
periods, phase reversal sequences and duty cycles.
25. The method of claim 24, wherein said grating periods are
selected to define the location of said at least two dominant
Fourier components.
26. The method of claim 20, further comprising providing a chirp in
said quasi-phase-matching grating.
27. The method of claim 20, further comprising apodizing said
dominant Fourier components to eliminate higher harmonics.
28. The method of claim 19, further comprising in-coupling and
out-coupling light into and out of said quasi-phase-matching
grating.
29. The method of claim 19, wherein said optical frequency mixing
comprises at least one mixing operation selected from the group
consisting of second harmonic generation, difference frequency
generation, sum frequency generation, and parametric
amplification.
30. The method of claim 19, wherein said optical frequency mixing
comprises a cascaded optical frequency mixing.
31. The method of claim 19, wherein light comprising at least two
long wavelength beams is in-coupled into said quasi-phase-matching
grating and said optical frequency mixing is performed
simultaneously on said at least two long wavelength beams.
32. A method for engineering a multi-channel optical frequency
mixer comprising: a) providing a non-linear optical material having
an effective nonlinearity d.sub.eff; b) determining at least two
short wavelength channels; and c) producing a quasi-phase-matching
grating in said non-linear optical material to define a spatial
distribution of said effective nonlinearity d.sub.eff, such that
said at least two short wavelength channels are quasi-phase-matched
for performing optical frequency mixing. wherein said
quasi-phase-matching grating is produced by selecting a Fourier
transform of said spatial distribution to the spatial frequency
domain to define at least two dominant Fourier components
corresponding to said at least two short wavelength channels.
33. (canceled)
34. The method of claim 32, wherein said selecting is performed by
setting at least one parameter of said quasi-phase-matching grating
selected from the group consisting of grating period, phase
reversal sequence, and duty cycle.
35. A multi-channel optical frequency mixer produced by the method
of claim 32.
36. A multi-channel optical frequency mixer for all-optical signal
processing using at least two long wavelength beams, said
multi-channel optical frequency mixer comprising: a) a nonlinear
optical material having an effective nonlinearity d.sub.eff; b) a
quasi-phase-matching grating defining a spatial distribution of
said effective nonlinearity d.sub.eff in said nonlinear optical
material, such that a Fourier transform of said spatial
distribution to the spatial frequency domain defines at least two
short wavelength channels quasi-phase-matched for performing
optical frequency mixing.
37. The multi-channel optical frequency mixer of claim 36, wherein
said Fourier transform of said spatial distribution comprises at
least two dominant Fourier components corresponding to said at
least two short wavelength channels.
38. The multi-channel optical frequency mixer of claim 37, wherein
said Fourier transform of said spatial distribution comprises an
even number of said dominant Fourier components.
39. The multi-channel optical frequency mixer of claim 37, wherein
said Fourier transform of said spatial distribution comprises an
odd number of said dominant Fourier components.
40. The multi-channel optical frequency mixer of claim 37, wherein
said quasi-phase-matching grating has predetermined grating
parameters for producing said at least two dominant Fourier
components.
41. The-multi-channel optical frequency mixer of claim 40, wherein
said predetermined grating parameters are selected from the group
consisting of grating periods, phase reversal sequences and duty
cycles.
42. The multi-channel optical frequency mixer of claim 40, wherein
said grating has a uniform grating period superposed by a phase
reversal sequence.
43. The multi-channel optical frequency mixer of claim 42, wherein
said phase reversal sequence has a predetermined duty cycle.
44. The multi-channel optical frequency mixer of claim 37, wherein
said quasi-phase-matching grating further comprises a chirp.
45. The multi-channel optical frequency mixer of claim 37, further
comprising optical structures for in-coupling and out-coupling
light into and out of said quasi-phase-matching grating.
46. The multi-channel optical frequency mixer of claim 36, further
comprising at least one waveguide.
47. The multi-channel optical frequency mixer of claim 46, wherein
said quasi-phase-matching grating is distributed within said at
least one waveguide.
48. The multi-channel optical frequency mixer of claim 46, further
comprising a mode controlling structure.
49. The multi-channel optical frequency mixer of claim 36, wherein
said nonlinear optical material comprises a substrate having at
least one component selected from the group consisting of lithium
niobate, lithium tantalate, MgO:LiNbO.sub.3, Zn:LiNbO.sub.3,
MgO:LiTaO.sub.3, stoichiometric lithium niobate, stoichiometric
lithium tantalate, potassium niobate, KTP, KTA, RTA, RTP and
members of the III-V semiconductor family.
50. The multi-channel optical frequency mixer of claim 49, further
comprising a waveguide in or on said substrate.
51. The multi-channel optical frequency mixer of claim 50, wherein
said waveguide is an in-diffused waveguide.
52. The multi-channel optical frequency mixer of claim 36, further
comprising a polarization control system for rendering said
multi-channel optical frequency mixer polarization diverse.
53. The multi-channel optical frequency mixer of claim 52, wherein
said polarization control system comprises at least one element
selected from the group consisting of polarization mode separator,
polarization rotator, optical isolator, optical circulator, optical
fiber, polarization maintaining fiber and polarization controller.
Description
RELATED APPLICATIONS
[0001] This application claims priority from Provisional Patent
Application 60/206,874 filed on May 24, 2000 and from Provisional
Patent Application entitled "Multiple Channel Optical Frequency
Mixers for All-Optical Signal Processing" filed on Feb. 1, 2001,
both of which are incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to multi-channel
optical frequency mixers using quasi-phase-matching for all-optical
signal processing, and in particular to quasi-phase-matching
gratings engineered to have multiple quasi-phase-matched channels
for performing frequency mixing operations.
BACKGROUND OF THE INVENTION
[0003] The drive for robust, high-capacity information networks has
resulted in many advances in the field of optical signal routing
and processing. While most local networks still rely on
electronics, many long distance communications lines are using
optical signals to transmit information. Depending on the
transmission protocols selected and transmission characteristics of
the optical components used, the information-bearing optical
signals are contained in a number of channels at predetermined
optical frequencies. There are numerous protocols for defining
channel parameters, including Wavelength Division Multiplexing
(WDM) or Dense Wavelength Division Multiplexing (DWDM) protocols.
The waveguides used in these long distance networks are optical
fibers, which offer advantages such as low loss, immunity to
interference and, most importantly, an extremely large
bandwidth.
[0004] To transmit information, data is modulated on optical
carrier a signals of wavelengths corresponding to the selected
channels (e.g., WDM channels). The information-bearing carrier
signals are combined at the transmitting end and sent via the
optical fiber to the receiving end. Along the way, the signals
encounter various active and passive network elements including
routing nodes, frequently equipped with repeaters and dispersion
compensation elements among others. Traditionally, at many of these
nodes the signals are converted back into electronic form for
processing. Afterwards, they are converted back into optical
signals as they leave the node. Speed, bandwidth and power
requirements can be limiting due to this conversion.
[0005] The above problems are circumvented in an all-optical
network in which the nodes switch optical signals in the different
wavelength channels in different directions generally without
converting the optical signals into electronic form. Several
concepts for all-optical WDM networks have been developed for this
purpose. The fundamentals of all-optical routing operations require
the ability to discriminate between two signals of wavelengths
.lambda..sub.1 and .lambda..sub.2 and to switch them to different
optical paths based on their wavelengths. Switches which can
perform such operations are known in the art and include, among
other, acousto-optically or electro-optically tunable filters and
micro-electromechanical systems (MEMS). In addition, all-optical
switches should also be able to perform wavelength conversion
functions, i.e., switch the two optical signals between different
optical carrier wavelengths, either within the immediate network or
when transferring to a neighboring network. Such wavelength
switches can be used to build wavelength interchangers or
wavelength interchanging cross-connects. More information about
such switches is provided by S. J. B. Yoo in "Wavelength Conversion
Technologies for WDM Network Applications", Journal of Lightwave
Technology, Vol. 14, No. 6, June 1996, pp. 955-66 as well as in
U.S. Pat. No. 5,825,517 to Antoniades et al. and in the references
cited therein.
[0006] In a practical all-optical network the nodes have to be able
to perform frequency mixing operations on a large number of optical
signals of different wavelengths, i.e., multiple signals contained
in different channels. One way to achieve frequency mixing
operations on a number of signals at multiple wavelengths is to
employ separate discrete single channel devices. Typically, single
channel frequency mixing devices employ an optical material
exhibiting a nonlinear susceptibility to perform one or more
frequency mixing operations. Among other, such operations can
include second harmonic generation (SHG), difference frequency
generation (DFG), sum frequency generation (SFG), or parametric
amplification. For example, it is sometimes useful to perform SHG
followed by DFG, which uses the second harmonic generated by SHG.
General information about wavelength conversion in multiple WDM
channels is provided by Lacey, J. P. R. et al., in "Four-Channel
Polarization-Insensitive Optically Transparent Wavelength
Converter", IEEE Photonics Technology Letters, Vol. 9, No. 10,
October 1997, pp. 1355-7.
[0007] To achieve efficient frequency conversion many devices use
quasi-phase-matching (QPM) to counteract the phase slip between the
generating nonlinear polarization and the generated or converted
optical field as these propagate through the nonlinear optical
material. Thus, there is a phase velocity mismatch between the
generating polarization and generated optical signals. QPM employs
a grating in the nonlinear material to periodically compensate for
this phase velocity mismatch. There are several methods for
producing and tuning such QPM gratings and general information on
the theory and applications of QPM within optical waveguides can be
found in Michael L. Bortz's Doctoral Dissertation entitled
"Quasi-Phasematched Optical Frequency Conversion in Lithium Niobate
Waveguides", Stanford University, 1995 as well as M. L. Bortz et
al., "Increased Acceptance Bandwidth for Quasiphasematched Second
Harmonic Generation in LiNbO.sub.3 Waveguides", Electronics
Letters, Vol. 30, Jan. 6, 1994, pp. 34-5.
[0008] Several prior art references teach the use of QPM for
purposes of phasematching signals with do not bear information. For
example, U.S. Pat. No. 5,644,584 to Nam et al.; U.S. Pat. No.
5,912,910 to Sanders et al.; U.S. Pat. No. 6,021,141 to Nam et al.
and Becouarn, L. et al., "Cascaded Second-Harmonic and
Sum-Frequency Generation of a CO.sub.2 Laser Using a Single
Quasi-Phase-Matched GaAs Crystal", Conference on Lasers and
Electro-Optics, IEEE, Vol. 6, pp. 146-7, 1998 teach conversion of
output signals from lasers and conversion of optical signals which
do not carry information.
[0009] Meanwhile, specific application of QPM based wavelength
converters dealing with information-bearing signals and including
WDM applications are discussed by C. Q. Xu et al., "1.5 Jim Band
Efficient Broadband Wavelength Conversion by Difference Frequency
Generation in a Periodically Domain-Inverted LiNbO.sub.3 Channel
Waveguide", Applied Physics Letters, Vol. 63, 27 December 1993, pp.
3559-61; C. Q. Xu et al., "Efficient Broadband Wavelength Converter
for WDM Optical Communication Systems", Conference on Optical Fiber
Communication, IEEE, 20-25 February 1994; M. H. Chou et al.,
"1.5-.mu.m-Band Wavelength Conversion Based on Cascaded
Second-Order Nonlinearity in LiNbO.sub.3 Waveguides", IEEE
Photonics Technology Letters, Vol. 11, No. 6, June 1999, pp. 653-5;
as well as M. H. Chou et al., "1.5-.mu.m-Band Wavelength Conversion
Based on Difference-Frequency Generation in LiNbO.sub.3 Waveguides
with Integrated Coupling Structures", Optics Letters, Vol. 23, No.
13, Jul. 1 1998, pp. 1004-6. In addition, U.S. Pat. No. 5,434,700
to Yoo teaches an all-optical wavelength converter which uses an
optical waveguide with regions having differing nonlinear optical
susceptibilities such that the regions form a quasi-phase-matching
grating. This single channel device is proposed for use in optical
WDM networks to convert a single signal frequency.
[0010] Further, U.S. Pat. No. 5,815,307 to M. Arbore et al., and
U.S. Pat. No. 5,867,304 to Galvanauskas et al. teach the use of
aperiodic QPM gratings. In particular, these references teach the
use of aperiodic QPM gratings in nonlinear materials for
simultaneous frequency conversion and compression of optical
pulses.
[0011] Unfortunately, setting up a number of single channel devices
to perform frequency mixing operations on a number of signals in
parallel is usually impractical and introduces excessive losses in
the network. This is especially true when the number of channels or
wavelengths is large, e.g., in the case of DWDM. Hence, it would be
a significant advance to provide an apparatus and method for
performing frequency mixing operations on signals in many
wavelength channels simultaneously without having to use a number
of dedicated single channel devices. Specifically, it would be very
useful to have such apparatus tuned for frequency mixing operations
using more than one short wavelength signals by having
corresponding short wavelength channels.
OBJECTS AND ADVANTAGES
[0012] In view of the above, it is a primary object of the present
invention to provide a multi-channel optical frequency mixer for
frequency mixing operations. In particular, the frequency mixer is
to be quasi-phase-matched to at least two short wavelength channels
for performing these mixing operations.
[0013] It is also an object of the invention to provide a method
for defining a quasi-phase-matching grating to achieve
quasi-phase-matching in a number of short wavelength channels.
[0014] Yet another object of the invention is to provide a
multi-channel optical frequency mixer and methods for engineering
such mixers for phasematching wavelengths whose location and
spacing is defined by the International Telecommunication Union
(ITU) standards.
[0015] It is an additional object of the invention to provide a
multi-channel optical frequency mixer which can be employed in
devices such as a multiple channel add/drop, a multiple channel
switch and a multiple channel optical sampler. The multi-channel
mixer of the invention should likewise be adaptable to performing
wavelength broadcasting wherein each of a number of input signals
can be simultaneously converted into a number of output
wavelengths. The multi-channel mixer should enable broadcasting by
simultaneous utilization of multiple short wavelength channels.
[0016] Still another object of the invention is to ensure that the
multi-channel optical mixer and engineering methods of the
invention can be employed in optical networks such as WDM, DWDM,
TDM and other networks.
[0017] Yet an additional object is to provide a multi-channel
optical frequency mixer having the property of
polarization-insensitive operation.
[0018] These and numerous other advantages of the present invention
will become apparent upon reading the detailed description.
SUMMARY
[0019] In response to the objects set forth above, the present
invention provides a multi-channel optical frequency mixer for
all-optical signal processing. The multi-channel mixer has a
nonlinear optical material exhibiting an effective nonlinearity
d.sub.eff. Further, the multi-channel mixer has a
quasi-phase-matching grating defining a spatial distribution of the
effective nonlinearity d.sub.eff in the nonlinear optical material.
The spatial distribution is defined in such a manner that a Fourier
transform of it to the spatial frequency domain defines at least
two short wavelength channels which are quasi-phase-matched for
performing optical frequency mixing.
[0020] The Fourier transform of the spatial distribution is such
that it has at least two dominant Fourier components corresponding
to the at least two short wavelength channels. In one embodiment,
the Fourier transform of the spatial distribution has an even
number of dominant Fourier components. In another embodiment, the
Fourier transform of the spatial distribution has an odd number of
dominant Fourier components.
[0021] The quasi-phase-matching grating, which can include an
abrupt or continuous spatial variation of d.sub.eff, has
predetermined grating parameters selected to produce the at least
two dominant Fourier components. The grating parameters which are
appropriately chosen to produce the desired Fourier transform are
the local grating periods, phase reversal sequences and duty
cycles. In one embodiment a grating with a uniform grating period
superposed by a phase reversal sequence with a 50% duty cycle is
used to produce a Fourier transform with two dominant Fourier
components and hence two quasi-phase-matched short wavelength
channels for all-optical signal processing. In another embodiment a
grating with a uniform grating period superposed by a phase
reversal sequence with a 26.5% duty cycle is used to produce a
Fourier transform with three equal dominant Fourier components and
thus three quasi-phase-matched channels.
[0022] In a preferred embodiment, the multi-channel mixer has
optical structures for in-coupling and out-coupling light into and
out of the quasi-phase-matching grating. It is further preferred
that the multi-channel mixer have at least one waveguide and that
the quasi-phase-matching grating be distributed within that
waveguide. The multi-channel mixer can be further equipped with a
mode controlling structure for controlling the mode of light
admitted into the waveguide.
[0023] The multi-channel mixer is fabricated in a substrate of
nonlinear optical material. The nonlinear optical material is
selected, among other, for its second order susceptibility
.chi..sup.(2) enabling it to perform the frequency mixing
operations. Thus, multi-channel mixer of the invention can perform
any desired nonlinear optical frequency mixing operation. These
nonlinear operations include second harmonic generation (SHG),
difference frequency generation (DFG), sum frequency generation
(SFG) and parametric amplification. Suitable nonlinear optical
materials for performing these operations include one or more
components selected among lithium niobate, lithium tantalate,
MgO:LiNbO.sub.3, Zn:LiNbO.sub.3, MgO:LiTaO.sub.3, stoichiometric
lithium niobate, stoichiometric lithium tantalate, potassium
niobate, KTP, isomorphs of KTP such as KTA, RTA, RTP, as well as
GaAs and other members of the III-V semiconductor family. Of
course, other suitable nonlinear optical materials can also be used
in the multi-channel mixer of the invention.
[0024] As noted above, it is preferred that the mixer have a
waveguide fabricated in or on the substrate made of the nonlinear
optical material. For example, the waveguide is an in-diffused
waveguide produced by a suitable method, as will be known to those
skilled in the art.
[0025] The multi-channel mixer of the invention can have a
polarization control system for rendering it polarization diverse.
The polarization control system is typically made of several
components selected among elements such as polarization mode
separators, polarization rotators, optical isolators, optical
circulators, optical fibers, polarization maintaining fibers and
polarization controllers.
[0026] In accordance with the method of invention the spatial
distribution of the effective nonlinearity d.sub.eff in the
nonlinear optical material is defined by the quasi-phase-matching
grating. The Fourier transform of the spatial distribution defines
at least two short wavelength channels quasi-phase-matched for
performing optical frequency mixing. At least two dominant Fourier
components correspond to these at least two short wavelength
channels. Specifically, grating parameters such as grating periods,
phase reversal sequences and duty cycles are set to produce these
at least two dominant Fourier components. Appropriate choice of
phase reversal sequence or sequences is used to set the number of
dominant Fourier components. The grating periods are selected to
define the location of the dominant Fourier components.
[0027] The light can be in-coupled and out-coupled of the
quasi-phase-matching grating using appropriate optical structures
(e.g., lenses, wave guide mode filters, waveguide tapers, waveguide
directional couplers etc.). Typically, the light comprises one or
more beams. For the purpose of all-optical signal processing, one
or more of these beams can be impressed with information.
[0028] In some embodiments the second order susceptibility of the
nonlinear optical material is used twice in cascaded optical
frequency mixing; .chi..sup.(2):.chi..sup.(2) (cascaded mixing per
se being known to those skilled in the art). These schemes allow
one to perform two frequency mixing operations in the same
quasi-phase-matching grating (e.g., SHG and DFG).
[0029] It is also noted that the light in-coupled into the
quasi-phase-matching grating can comprise at least two long
wavelength beams. In these situations, the optical frequency mixing
can be performed simultaneously on the long wavelength beams.
[0030] Thus, in general, the invention provides a method for
engineering multi-channel mixers by selecting the spatial
distribution of the effective nonlinearity d.sub.eff of the
nonlinear optical material such that at least two short wavelength
channels are quasi-phase-matched for performing optical frequency
mixing.
[0031] A detailed description of the invention and the preferred
and alternative embodiments is presented below in reference to the
attached drawing figures.
BRIEF DESCRIPTION OF THE FIGURES
[0032] FIG. 1A is a diagram illustrating the principles of
quasi-phase-matched nonlinear mixing in a single channel optical
frequency mixer in accordance with the prior art.
[0033] FIG. 1B is a diagram illustrating second harmonic generation
(SHG) and difference frequency generation (DFG) using the single
channel optical frequency mixer of FIG. 1A in accordance with the
prior art.
[0034] FIG. 2 is a diagram illustrating the fundamental concepts of
using the Fourier transform for engineering multi-channel optical
frequency mixers in accordance with the invention.
[0035] FIG. 3A is a diagram illustrating a two-channel mixer
obtained by a superposition of a phase reversal sequence on a QPM
grating with a uniform grating period.
[0036] FIG. 3B is a graph of the Fourier transform of the
superposition of the phase reversal sequence and grating of the
two-channel mixer of FIG. 3A.
[0037] FIG. 4 illustrates difference frequency generation (DFG)
using two wavelength channels produced by the grating of FIG.
3A.
[0038] FIG. 5 is a diagram illustrating the superposition of a
phase reversal sequence with a 26.5% duty cycle on a QPM grating
with a uniform grating period.
[0039] FIG. 6 is an isometric view of a multi-channel frequency
mixer in accordance with the invention.
[0040] FIG. 7 is a generalized multi-channel mixer in accordance
with the invention.
[0041] FIG. 8A-D illustrates devices employing multi-channel mixers
of the invention for WDM purposes.
[0042] FIG. 9 shows a multi-channel mixer with a polarization
control system for rendering it polarization insensitive.
[0043] FIG. 10 are graphs illustrating SHG conversion efficiencies
for 2-, 3- and 4-channel mixers in accordance with the
invention.
DETAILED DESCRIPTION
THEORY REVIEW AND PRIOR ART DISCUSSION
[0044] The method of the invention will be best understood by first
reviewing the theory of quasi-phase-matching based on prior art
quasi-phase-matching grating 10 of FIG. 1A. Grating 10 is a uniform
quasi-phase-matching (QPM) grating 10 of length L and is fabricated
in a nonlinear optical material 12. Material 12 has a second order
nonlinear susceptibility .chi..sup.(2) enabling it to perform
optical frequency mixing operations. The nonlinear susceptibility
of material 12 is characterized by a spatial distribution of
nonlinearity in material 12. In single-domain bulk form of material
12 the distribution is described by a nonlinear coefficient
d.sub.o.
[0045] In the present case, the spatial distribution of the
nonlinearity varies in a manner conveniently described with the aid
of normalized nonlinearity distributions. As shown in transverse
cross section or slice 16 of material 12, the nonlinearity has a
normalized nonlinearity distribution d(x,y) in the x-y plane with a
value normalized to range from 0 to 1. Further, the nonlinearity
has a normalized nonlinearity distribution d(z) along the z axis
normalized to range from 1 to -1. (It is noted that in some cases
such factorization of the nonlinearity distribution to d(x,y) and
d(z) may not be possible). Here, the z-axis is conveniently chosen
as the direction along which optical frequency mixing is performed
(direction of light propagation).
[0046] The nonlinear coefficient d.sub.o expressed with the aid of
its normalized nonlinearity distributions is related to the second
order nonlinear susceptibility .chi..sup.(2) by:
.chi..sup.(2)=2d.sub.od(x,y)d(z).
[0047] QPM grating 10 has a number of regions 14 of alternating
sign of nonlinear susceptibility .chi..sup.(2) as indicated by the
arrows. This is easily accomplished by engineering the nonlinearity
such that the sign of the normalized nonlinearity distribution d(z)
in adjacent regions 14 alternates between -1 and 1. Methods for
engineering the nonlinearity to achieve such distribution d(z) are
known in the art. For example, if material 12 is a ferroelectric it
can be periodically poled. A person skilled in in the art will be
familiar with numerous other methods for engineering the
nonlinearity depending on the type of material 12.
[0048] Light waves of different frequencies traveling through
nonlinear optical material 12 experience phase slip with respect to
one another. This is because they see different indices of
refraction in material 12 causing them to propagate at different
phase velocities. In other words, they experience a phase velocity
mismatch. Nonlinear optical frequency mixing involves a driving
nonlinear polarization and interacting light waves at two or more
frequencies and is thus affected by phase slip. QPM grating 10
periodically counteracts the effects of the phase slip because the
second order susceptibility .chi..sup.(2) in adjacent regions 14 is
engineered to alternate in sign. Specifically, the thickness of
regions 14 is such that when the driving polarization and
interacting waves have slipped off by .pi., as it happens over a
certain distance of travel referred to as the coherence length
L.sub.c, they enter into the next region 14 with reversed sign of
linear susceptibility .chi..sup.(2). In other words, the thickness
of regions 14 is set to the value of the coherence length L.sub.c.
Consequently, the driving polarization and interacting waves which
slip off by .pi. over coherence length L.sub.c and would, due to
their out-of-phase relationship, reverse the nonlinear frequency
mixing operation over the next coherence length L.sub.c (thus
undoing the results of frequency mixing performed over the first
coherence length L.sub.c), continue to perform the desired
frequency mixing operation in the subsequent region 14. Based on
this, it is also clear that QPM grating 10 should strive for a
large number of regions 14 (i.e., large length L) to increase the
efficiency of nonlinear mixing.
[0049] It is further useful to consider the action of QPM grating
10 during a particular nonlinear mixing process involving light of
three different frequencies (three-wave mixing). This process can
be a difference frequency generation (DFG) operation involving a
short wavelength beam 18, frequently referred to in such cases as a
pump beam, and a long wavelength beam 20, frequently referred to
such cases as a signal beam, and an output beam 22, which is also
at a long wavelength. Short wavelength beam 18 is defined by an
electric field E.sub.p propagating at an angular frequency
.omega..sub.p and having a corresponding wave vector k.sub.p. Long
wavelength beam 20 is defined by an electric field E.sub.s at an
angular frequency .omega..sub.s and a wave vector k.sub.s.
Similarly, output beam 22 is defined by an electric field E.sub.out
at an angular frequency .omega..sub.out and a wave vector
k.sub.out. The phase mismatch .DELTA.k of these three beams 18, 20,
22 is counteracted by a grating vector k.sub.g of QPM grating 10
related to regions 14 via the period .LAMBDA..sub.g
(k.sub.g=2.pi./.LAMBDA..sub.g) as follows: 1 k p - k s - k out = 2
( n p p - n s s - n out out ) = k = k g .
[0050] In this equation n.sub.p, n.sub.s, n.sub.out are the
respective indices of refraction experienced by beams 18, 20, 22 at
their respective frequencies, here expressed in terms of their
corresponding wavelengths .lambda..sub.p, .lambda..sub.s,
.lambda..sub.out.
[0051] When pump beam 18 and signal beam 20 enter QPM grating 10
they start to generate output beam 22 by DFG using the second order
nonlinear susceptibility .chi..sup.(2) of material 12, as shown in
FIG. 1B. (A person skilled in the art will recognize that other
nonlinear mixing processes also take place within material 12.
These are not discussed at this point for reasons of clarity.) The
nonlinear mixing process is driven by the nonlinear polarization
P.sub.NL set up in material 12, as illustrated in slice 16 of
material 12 in FIG. 1A. Disregarding the dispersive nature of
nonlinear susceptibility .chi..sup.(2) nonlinear polarization
P.sub.NL is established in proportion to the nonlinear
susceptibility .chi..sup.(2) and also in proportion to the square
of the total electric field E.sup.2 of all three interacting waves
or beams 18, 20 and 22. This relationship can be expressed as: 2 P
NL = 1 2 ( 2 ) 0 E p 2 ,
[0052] where .epsilon.hd o is the permittivity of free space. As
these three beams 18, 20 and 22 propagate through material 12, QPM
grating 10 does not allow nonlinear polarization P.sub.NL driving
the frequency conversion process and the beams to slip out of phase
by any more than .pi., as explained above. Hence, efficient
generation of output beam 22 at angular frequency .omega..sub.out
takes place over length L of QPM grating 12.
[0053] From the above equations, it can be shown that the portion
of nonlinear polarization P.sub.NL,out responsible for DFG
generation of output beam 22 is described by:
P.sub.NL,out=2d.sub.od(x,y)d(z).epsilon..sub.oE.sub.pE.sub.s*,
[0054] where the asterisk denotes the conjugate of electric field
E.sub.s of long wavelength beam 20. This process is visualized in
FIG. 1B, where it is seen that short wavelength beam 18 at
.omega..sub.p mixes with long wavelength beam 20 at .omega..sub.s
to produce output beam 22 at .omega..sub.out "mirrored" with
respect to half the pump frequency .omega..sub.p/2 by DFG. Thus,
output beam 22 depends on the conjugate electric field E*.sub.s of
electric field E.sub.s of long wavelength beam 20.
[0055] It should be noted that the DFG conversion has a
predetermined efficiency less than 100% and thus the intensity of
output beam 22 is lower than that of long wavelength beam 20. (In
fact, under most conditions the output power, P.sub.out is
proportional to the product of pump power and signal power.) It
should also be noted that same QPM grating 10 can be used to
perform second harmonic generation (SHG) of short wavelength beam
18 at .omega..sub.p by using a long wavelength beam 24 at
.omega..sub.p/2. In this case, long wavelength beam 24 at
.omega..sub.p/2 is commonly referred to as the pump. The SHG
process is well-known and also indicated in FIG. 1B. The
quasi-phase-matching condition for SHG is:
k.sub..omega..sub..sub.p=2k.sub..omega..sub..sub.p/2-k.sub.g.
[0056] QPM grating 10 has a wide tuning range or bandwidth BW.sub.s
for performing DFG using short wavelength beam 18. For example,
long wavelength beam 20 can be substituted by another long
wavelength beam 20' having an angular frequency .omega.'.sub.s
substantially larger than .omega..sub.s, and a wave vector k'.sub.s
correspondingly larger than wave vector k.sub.s of beam 20. Now,
output beam 22' is reflected about .omega..sub.p/2 by DFG to a
lower angular frequency .omega.'.sub.out with a correspondingly
smaller wave vector k'.sub.out than output beam 22. Thus, the DFG
process using short wavelength beam 18 remains substantially
quasi-phase-matched by QPM grating 10. In other words, because the
wave vectors of input and output beams 20, 22 change in opposite
sense grating vector k.sub.g still approximately satisfies the
condition that:
k.sub.p-k.sub.s.sup.'-k.sub.out.sup.'=k.sub.g.
[0057] By virtue of this property of QPM grating 10, tuning
bandwidth BW, for long wavelength beam 20 when performing DFG with
a fixed short frequency beam 18 is typically on the order of tens
of nanometers in wavelength.
[0058] Unfortunately, the same is not true for a tuning range or
bandwidth BW.sub.p for short wavelength beam 18. In general, short
wavelength beam 18 can only be tuned over a very narrow 5 bandwidth
(typically only a few nm or less) while still maintaining the
quasi-phase-matching condition in QPM grating 10. In other words,
only one narrow short wavelength channel defined by bandwidth
BW.sub.p is available for short wavelength beam 18. In this sense
QPM grating 10 employed in nonlinear material 12 can only yield a
single short wavelength channel optical frequency mixer. Such
single channel mixer has only limited usefulness for optical signal
processing, e.g., all-optical processing, as already remarked in
the background section.
EMBODIMENTS OF THE INVENTION
[0059] In accordance with the invention, a multi-channel optical
frequency mixer 50, as shown in FIG. 2, is made in a nonlinear
optical material 52. Material 52 is selected for its second order
nonlinear susceptibility .chi..sup.(2) as well as other material
properties known to a person skilled in the art to be used for the
desired optical frequency mixing operation or operations. Materials
which can be used in optical material 52 can be selected from among
lithium niobate, lithium tantalate, MgO:LiNbO.sub.3,
Zn:LiNbO.sub.3, MgO:LiTaO.sub.3, stoichiometric lithium niobate,
stoichiometric lithium tantalate, potassium niobate, KTP, isomorphs
of KTP such as KTA, RTA, RTP, as well as GaAs and other members of
the III-V semiconductor family. A person skilled in the art will
realize that numerous other materials and groups of materials
exhibiting suitably large nonlinear susceptibility .chi..sup.(2)
and other advantageous material properties for optical frequency
mixing are available and can be used in optical material 52.
[0060] A quasi-phase-matching grating 54 defines a spatial
distribution of an effective nonlinearity d.sub.eff. QPM grating 54
is engineered to yield a particular Fourier transform of the
effective nonlinearity d.sub.eff. Specifically, the spatial
distribution of the effective nonlinearity d.sub.eff is defined by
QPM grating 54 in such manner that the Fourier transform of that
spatial distribution to the spatial frequency domain defines at
least two short wavelength channels 56, 58 which are
quasi-phase-matched for performing optical frequency mixing.
[0061] Grating 54 has a number of regions 64 in which the
normalized nonlinearity distribution d(z) has a different magnitude
or sign. For example, the normalized nonlinearity distribution d(z)
in adjacent regions 64 exhibits a sign reversal. Regions 64 do not
form a grating with a single uniform grating period .LAMBDA..sub.g.
In fact, grating 54 is made up of several components. In the
embodiment shown, grating 54 has a grating period .LAMBDA..sub.g
with a 50% duty cycle superposed by a first phase reversal sequence
.PI..sub.1 of period .LAMBDA..sub.phase1 with a 50% duty cycle and
by a second phase reversal sequence .PI..sub.2 of period
.LAMBDA..sub.phase2 also with a 50% duty cycle.
[0062] Conveniently, the superposition of grating period
.LAMBDA..sub.g by phase reversal sequences with periods
.LAMBDA..sub.phase1 and .LAMBDA..sub.phase2 can be defined in terms
of an effective nonlinearity d.sub.eff along the z-direction
as:
d.sub.eff(z).ident.d.sub.od(x,y)G.sub.m,
[0063] where G.sub.m is a Fourier coefficient of the Fourier
decomposition of d.sub.eff(z) and d.sub.o is the effective
nonlinear coefficient of bulk material 52. It is known in the art
of mathematics that periodic functions can be Fourier decomposed
into a Fourier series. It is also known that Fourier series can be
appropriately chosen to produce certain desired functions. The
components of the Fourier series exist in an adjoint space. In the
case of the spatial distribution of d.sub.eff defined by grating
54, the adjoint space is the spatial frequency domain. The Fourier
transform of the effective nonlinearity d.sub.eff defined by
grating 54 thus defines Fourier components in the spatial frequency
domain.
[0064] In free space spatial frequencies associated with light
waves are conveniently characterized by wave vectors k. Inside
nonlinear material 52, however, wave vectors k are replaced by
propagation constants .beta., which vary with angular frequency
.omega. of the wave, i.e., .beta.=.beta.(.omega.). That is because
within nonlinear material 52 propagation constant .beta.
experiences dispersion. The group velocity v.sub.g of any light
wave of bandwidth .DELTA..omega. in medium 52 can be expressed as:
3 v g = ( d d ) c ,
[0065] evaluated at central frequency .omega..sub.c of bandwidth
.DELTA..omega.. This linear relationship does not take into account
higher order dispersion terms and hence can only be used to the
extent that higher order terms in the relationship between .omega.
and .beta. can be neglected. In some nonlinear frequency mixing
processes, e.g., in interactions between two light beams whose wave
vectors k are near-degenerate or degenerate, the above linear
relationship will not be sufficient to establish the relationship
between .omega. and .beta.. That is because the linear terms will
cancel and hence the higher order terms will become important. In
these cases a Taylor expansion around the center angular frequency
hi can be performed to obtain the higher order terms and thus
obtain a sufficiently accurate relationship between .beta. and
.omega..
[0066] In cases where .beta. and .omega. are related by the linear
relationship, the Fourier components existing in the spatial
frequency domain are related to the temporal frequency domain,
i.e., they are related to angular frequencies w via the reciprocal
of group velocity, 1/v.sub.g, and in the case of a bandwidth
.DELTA..omega. they are related via 1/.DELTA.v.sub.g. A person
skilled in the art of mathematics will be able to derive the
appropriate relationship between .omega. and .beta. for cases where
higher order terms are important. Thus, the Fourier transform
corresponds to components 56, 58, 60 and 62 in the time frequency
domain, as will also be appreciated by a person skilled in the art
of mathematics. In fact, components 56, 58, 60 and 62 are the four
major or dominant Fourier 25 components corresponding to angular
frequencies .omega..sub.p1, .omega..sub.p2, .omega..sub.p3 and
.omega..sub.p4, as shown in FIG. 2. In other words, the tuning
curve defining the relative conversion efficiency .eta..sub.rel
versus angular frequency .omega. of pump beams in QPM grating 54
has four main peaks at 56, 58, 60 and 62.
[0067] Multi-channel mixer 50 is thus a four-channel device and is
capable of performing optical mixing operations with short
wavelength beams contained in the four short wavelength channels
centered at angular frequencies .omega..sub.p1, .omega..sub.p2,
.omega..sub.p3 and .omega..sub.p4. In other words, grating 54
ensures that the quasi-phase-matching condition is satisfied for
optical frequency mixing operations which use short wavelength
beams at these four frequencies.
[0068] FIGS. 3A and 3B show in more detail the engineering of a
two-channel optical mixer 75 using a QPM grating 70 in accordance
with the invention. Referring to FIG. 3A, it is shown that QPM
grating 70 is obtained by superposing a phase reversal sequence 72
of period .LAMBDA..sub.phase with a substantially 50% duty cycle on
a uniform QPM grating 74 of period .LAMBDA..sub.g and a
substantially 50% duty cycle. By itself, uniform QPM grating 74
yields a single channel device. That is because the Fourier
transform of a uniform grating or, equivalently, of a periodic
square function, is a sinc function with a single dominant Fourier
component 76 corresponding to .omega..sub.p as indicated in FIG.
3B. Because grating 70 also contains phase reversal sequence 72,
the Fourier transform of QPM grating 70 is the convolution of the
sinc function representing the Fourier transform of grating 74 and
a comb function, in this case with two major peaks and a number of
minor peaks due to phase reversal sequence 72. This convolution
produces two dominant Fourier components 78, 80 corresponding to
angular frequencies .omega..sub.p1 and .omega..sub.p2. It should be
noted that to first order these two angular frequencies are evenly
spaced from the angular frequency .omega..sub.p of single-channel
grating 74. Thus, adding phase reversal sequence 72 has caused a
split of the dominant Fourier component 76 of uniform grating 74
into dominant Fourier components 78, 80.
[0069] The Fourier transform of QPM grating 70 also has a number of
peaks or higher order harmonics, generally indicated by 82. The
harmonics are due to the "squareness" of grating 74. These higher
order harmonics 82 are small in comparison to dominant Fourier
components 78, 80 and will generally not be relied upon for
performing optical frequency mixing. It will be understood by a
person skilled in the art that such higher order harmonics may be
generally eliminated by use of filter design techniques including,
but not limited to apodization of QPM grating 70. Meanwhile,
dominant Fourier components 78, 80 correspond to the two channels
centered at frequencies .omega..sub.p1 and .omega..sub.p2 of
two-channel QPM grating 70.
[0070] The operation of two-channel mixer 75 based on an exemplary
DFG process is illustrated in FIG. 4. As in FIG. 1B, light beams
are represented by arrows indicating beam intensities centered at
corresponding center angular frequencies. In contrast to prior art
devices, two-channel mixer 75 accepts two short wavelength beams
90, 92 at angular frequencies .omega..sub.p1 and .omega..sub.p2
corresponding to dominant Fourier components 78, 80. Since the
operation being performed is DFG short wavelength beams 90, 92 are
acting as pump beams in this case. Each beam 90, 92 has a tuning
bandwidth BW.sub.p1, BW.sub.p2 which is related to the associated
dominant Fourier component as the width of the spatial Fourier
transform scaled by .DELTA.v.sup.-1. When a long wavelength beam
94, in this case a signal beam, at angular frequency .omega..sub.s
is input into two-channel mixer 75 it produces a first output beam
96 at angular frequency .omega..sub.out1 by DFG with beam 90 via
nonlinear susceptibility .chi..sup.(2). Beam 94 can also produce a
second output beam 98 at angular frequency .omega..sub.out2 by DFG
with beam 92. (It should be noted that beams 90 and 92 do not need
to be present in mixer 75 simultaneously.) Hence, QPM grating 70
engineered according to the invention defines two short wavelength
channels, centered at .omega..sub.p1 and .omega..sub.p2,
quasi-phase-matched for performing optical frequency mixing, in
this case DFG.
[0071] Beams 90, 92 can be input into mixer 75 simultaneously or at
different times to perform DFG with long wavelength beam 94
simultaneously or at different times. Also, more than one long
wavelength beam can take advantage of the two pump beams for
optical frequency mixing operations. For example, two or more long
wavelength beams can be supplied to mixer 75 and the optical
frequency mixing can be performed simultaneously on these two or
more long wavelength beams. Of course, two-channel 20 mixer 75 can
also be used to perform other optical frequency mixing operations.
These nonlinear operations can involve second harmonic generation
(SHG), sum frequency generation (SFG) and parametric amplification.
It is also possible to perform several different mixing operations
in mixer 75 at the same time, e.g., SHG and DFG, as will be
appreciated by those skilled in the art. For example, this can be
done by using the second order susceptibility of the nonlinear
optical material twice in cascaded optical frequency mixing;
.chi..sup.(2):.chi..sup.(2). Cascaded schemes are known in the art
and allow one to perform two frequency mixing operations in the
same quasi-phase-matching grating (e.g., SHG and DFG).
[0072] Two-channel mixer 75 with QPM grating 70 has taken advantage
of the Fourier transform of phase reversal sequence 72 to "split"
the one short wavelength channel offered by QPM grating 74 into two
short wavelength channels. Referring back to QPM grating 54, the
superposition of two phase reversal sequences on a uniform grating
"splits" one short wavelength channel offered by the uniform
grating into four short wavelength channels. In fact, a person
skilled in the art of mathematics will recognize that any desired
even number of dominant Fourier components and hence even number of
short wavelength channels can be produced by a superposition of the
appropriate number of phase reversal sequences on a uniform
grating. Of course, a person skilled in the art will also be
familiar with the nature of the Fourier transform and appreciate
that there are many ways in which the spatial distribution of the
effective nonlinearity d.sub.eff can be engineered to produce an
even number of dominant Fourier components and hence short
wavelength channels.
[0073] In some embodiments an odd number of short wavelength
channels is required in multi-channel mixer 75. FIG. 5 illustrates
uniform grating 74 superposed by a phase reversal sequence 73 with
a duty cycle of approximately 26.5%. The Fourier transform of the
spatial distribution of a QPM grating produced by this
superposition has three equal amplitude dominant Fourier
components. Specifically, in addition to the two new dominant
Fourier components corresponding to .omega..sub.p1 and
.omega..sub.p2, it retains a dominant Fourier component
corresponding to the location of the original dominant Fourier
component of uniform grating 74, i.e., at .omega..sub.p. A person
skilled in the art of mathematics will recognize that by altering
the duty cycles of phase reversal sequences it is possible to
engineer QPM gratings with an odd number of dominant Fourier
components.
[0074] In some embodiments grating 74 can additionally contain a
chirp. The chirp can be produced in grating 74 to compress the
light by 10 counteracting phase dispersion during the frequency
mixing process. Techniques for chirping QPM gratings are known in
the art and a skilled artisan will find information on its
implementation, e.g., in U.S. Pat. No. 5,815,307 to M. Arbore et
al.
[0075] The QPM grating engineering techniques of the invention can
be used to make a variety of multi-channel mixers in various
configurations. FIG. 6 is an isometric view of a multi-channel
mixer 100 equipped with a QPM grating 102 provided in a substrate
104. Conveniently, entire substrate 104 is made of a nonlinear
optical material 101 or materials which are to perform optical
mixing operations expected of multi-channel mixer 100. Thus,
nonlinear optical material 101 can consist of one or more of
material components including without limitation, lithium
tantalate, MgO:LiNbO.sub.3, Zn:LiNbO.sub.3, MgO:LiTaO.sub.3,
stoichiometric lithium niobate, stoichiometric lithium tantalate,
potassium niobate, KTP, isomorphs of KTP such as KTA, RTA, RTP, or
GaAs or other members of the III-V semiconductor family as well as
any organic nonlinear materials and nonlinear polymers. A person
skilled in the art will recognize that the exact choice of material
depends on various considerations including the type of mixing
operations which will be performed in QPM grating 102. In fact,
even organic nonlinear materials and nonlinear polymers could be
used as material 101.
[0076] QPM grating 102 is made up of domains or regions 106
defining a spatial distribution of the effective nonlinearity
d.sub.eff. To achieve this, regions 106 can be formed by
appropriate growth of regions 106 to produce different non-linear
orientations in adjacent regions 106. Alternatively, regions 106
can be obtained by poling in cases when material 101 is a
ferroelectric material, a polymer or glass. A person skilled in the
art will appreciate that there are numerous techniques which can be
used to produce regions 106 as required for grating 102 depending
on the type of material 101 selected.
[0077] QPM grating 102 is distributed within a waveguide 110. The
use of waveguide 110 in material 101 is preferred because it aids
in guiding the interacting light beams and generally results in
better conversion efficiencies during the nonlinear optical mixing
operations as compared to bulk material. For example, waveguide 110
is fabricated within nonlinear optical material 101 after QPM
grating 102. When nonlinear optical material 101 is LiNbO.sub.3 or
LiTaO.sub.3 waveguide 110 may comprise waveguide structures that
include, without limitation, annealed proton exchanged (APE)
waveguides, buried waveguides, metal in-diffused waveguides
(including metals such as zinc, titanium, etc.) as will be
understood by those knowledgeable in the art.
[0078] Waveguide 110 has an input facet 112 and an output facet
114. In the present embodiment, input facet 112 and output facet
114 are located at opposing side walls of substrate 104. Input
facet 112 has an associated in-coupling or coupling element 116, in
this case a lens, for in-coupling light 118 into waveguide 110. An
out-coupling element 120 is provided past output facet 114 for
guiding output light 122 exiting through output facet 114. A person
skilled in the art will recognize that other coupling devices such
as tapers in waveguide 110 can be employed in conjunction with or
without a lens to serve the function of coupling elements 116 and
120. In general, coupling element 116 and coupling element 120 may
include without limitation optical elements such as optical fiber,
prism couplers, waveguide mode filters, waveguide couplers, and
tapered waveguide regions. In particular, mode controlling
structures for controlling the mode of light admitted into
waveguide 110 can be used to maximize the overlap of interacting
beams. As is known in the art, maximizing this overlap will ensure
high efficiency of the frequency mixing operations performed by
multi-channel mixer 100. A person skilled in the art will
appreciate that the best choice of coupling element 116 is made by
considering the wavelengths and modes of light which are to be
coupled into QPM grating 102.
[0079] In the present embodiment, substrate 104 also has a
waveguide 124 with an input facet 126 and an associated in-coupling
element 128 for in-coupling additional light 130. This arrangement
can be used when light 130 is not required for the nonlinear mixing
operation in first section of QPM grating 102 or if it can not be
efficiently in-coupled together with light 118 via in-coupling
element 116. Once again, coupling element 128 can include an
appropriate taper of the waveguide 124 and/or any of the optical
elements listed above.
[0080] Waveguide 124 is formed such that it extends next to and
parallel to waveguide 110 where QPM grating 102 is distributed.
This arrangement forms a coupling or junction 132 between
waveguides 124 and 110 and permits light 130 to be in-coupled via
the evanescent field into waveguide 110. A person skilled in the
art will recognize that junction 132 is merely one exemplary
structure for accomplishing this goal and that light 130 can be
in-coupled into waveguide 110 using other types of junctions which
may include without limitation, Y-junctions and directional
couplers.
[0081] QPM grating 102 is multi-channel. Specifically, QPM grating
102 is two-channel for quasi-phase-matching optical frequency
mixing operations which use two short wavelength channels
.omega..sub.p1 and .omega..sub.p2. Thus, QPM grating 102 is
analogous to QPM grating 70 discussed above.
[0082] During operation, in-coupling element 116 couples light 118
into waveguide 110 and QPM grating 102. In the present embodiment
light 118 contains two long wavelength beams at angular frequencies
.omega..sub.1, .omega..sub.2. Angular frequencies .omega..sub.1,
.omega..sub.2 are chosen to be half the frequencies of short
wavelength channels .omega..sub.p1 and .omega..sub.p2 respectively.
For illustrative purposes FIG. 6 shows only portions of these beams
in the form of pulses. It will be understood, however, that
continuous-wave beams can be used for any of these beams.
[0083] The first section of QPM grating 102 is used to generate two
second harmonics at .omega..sub.p1 and at .omega..sub.p2 of long
wavelength beams at .omega..sub.1, .omega..sub.2. It should be
noted long wavelength beams at .omega..sub.1, .omega..sub.2 play
the role of pump beams within the first section of QPM grating 102
when generating the second harmonics at .omega..sub.p1 and at
.omega..sub.p2. The two second harmonics, which are short
wavelength beams, continue to propagate into the second section of
QPM grating 102.
[0084] Light 130 in the form of two additional long wavelength
beams at .omega..sub.3 and .omega..sub.4 couples into waveguide 110
at junction 132. These two beams propagate into second section of
QPM grating 102 along with second harmonics at .omega..sub.p1 and
.omega..sub.p2. In second section of QPM grating 102 second
harmonics .omega..sub.s1 and .omega..sub.s2 obtained in the first
section of QPM grating 102 act as pump beams. Specifically, in the
second section they mix with long wavelength beams at
.omega..sub.3, .omega..sub.4 to produce output light 122 by DFG.
DFG between opit and .omega..sub.3, .omega..sub.4 respectively
generates output beams .omega..sub.out1, .omega..sub.out2 while DFG
between .omega..sub.p2, and .omega..sub.3, .omega..sub.4
respectively generates output beams .omega..sub.out3,
.omega..sub.out4. Output light 122 is out-coupled from
multi-channel mixer 100 via coupling element 120.
[0085] The power conversion performance of QPM grating 102 in the
small signal limit the output power can be expressed as: 4 P out
norm P S P P | 1 L 0 L .PI. ( z ) exp ( - j z ) z | 2 eq . 1
[0086] where P.sub..omega..sub..sup.P, P.sub..omega..sub..sup.S and
P.sub..omega..sub..sup.out are conventionally referred to as pump,
signal and converted output powers expressed in terms of their
angular frequencies. For example, in the first section of QPM
grating 102 during SHG generation of .omega..sub.P1
.omega..sub.p=.omega..sub.1 and .omega..sub.S=.omega..sub.1, and
P.sub..omega..sub..sup.out is the power of second harmonic
generated at .omega..sub.P1. For DFG generation of .omega..sub.out1
in the second section of QPM grating 102
.omega..sub.P=.omega..sub.P1 and .omega..sub.S=.omega..sub.3, and
P.sub..omega..sub..sup.out is the power of the DFG output beam at
.omega..sub.out1. .eta..sub.norm is the normalized efficiency in
units of W.sup.-1, which is proportional to the square of the
device length L (in this case the length of the first section of
QPM grating 102 for SHG and the length of second section of QPM
grating 102 for DFG) and the square of the modal overlap of the
interacting beams with the second-order optical nonlinearity
.chi..sup.(2) of material 101. The term .DELTA..beta. can be
expressed as:
.DELTA..beta.=2.pi.(n.sub.p/.lambda..sub.p-n.sub.s/.lambda..sub.s-n.sub.ou-
t/.lambda..sub.out-1/.LAMBDA..sub.g), eq. 2
[0087] where the refractive indices n are the effective indices at
the corresponding wavelengths .lambda., and .DELTA..beta.
represents the phase mismatch between the interacting waves and
uniform QPM grating 74 with superposed phase-reversal sequence 72
(period .LAMBDA..sub.phase). From this equation it is clear how
mismatch arises due to different effective indices of refraction
n.sub.p, n.sub.s and n.sub.out experienced in material 101 by pump,
signal and converted output frequencies, here expressed in terms of
their wavelengths .lambda..sub.p, .lambda..sub.s and
.lambda..sub.out. Finally, .PI.(z) is the superimposed
phase-reversal sequence 72.
[0088] In the particular case of QPM grating 102 phase-reversal
sequence 72 has a grating period of .LAMBDA..sub.phase and a duty
cycle of 50%. Thus, first phase-reversal sequence 72 can be
expressed as: 5 .PI. ( z ) = n = 1 .infin. ( 2 n ) [ exp ( jK n z )
+ exp ( - jK n z ) ] where K n = 2 n phase . eq . 3
[0089] Substituting the above expression for .PI.(z) into eq. 1
yields: 6 P out norm P P P S n = 1 , 3 , 5 .infin. ( 2 n ) 2 [ sin
c 2 ( + K n 2 L ) + sin c 2 ( - K n 2 L ) ] . eq . 4
[0090] For n=1 this equation results in a tuning curve with
phasematching frequencies corresponding to the two dominant Fourier
components (see FIG. 3B), as discussed above.
[0091] It will be clear to a person skilled in the art that the
embodiment in FIG. 6 illustrates only one exemplary multi-channel
optical mixer 100 which performs SHG and DFG using two short
wavelength channels. The generalized embodiment in FIG. 7
illustrates a multi-channel mixer 150 which can perform a number of
nonlinear mixing operations in series on various beams. Mixer 150
has a number of QPM gratings 152A, 152B, . . . , 152N engineered
according to the invention. It should be noted that although
gratings 152A, 152B, . . . , 152N are shown in the form of discrete
gratings, they can be substituted by non-discrete gratings. In
other words, gratings 152A, 152B, . . . , 152N can exhibit a
continuous variation in d.sub.eff (e.g., d.sub.eff(z) varies
continuously between -1 and 1). Input and output beams can be added
and retrieved beetween gratings 152A, 152B, . . . , 152N as
required with appropriate elements known in the art, e.g.,
directional couplers.
[0092] Mixer 150 accepts a number of input beams at frequencies
.omega..sub.in.sup.1 through .omega..sub.in.sup.x. For purposes of
all-optical signal processing any one of these signals can be
impressed with information. In fact, any beam can carry information
irrespective of whether it is an input beam at a short wavelength
corresponding to the short wavelength channel of the particular QPM
grating or is a long wavelength beam. Thus, in any frequency mixing
operation the beam carrying the information can be the pump beam or
the signal beam or both. Methods for modulating information on
optical beams are well-known in the art.
[0093] FIGS. 8A-D show several example applications of
multi-channel mixers according to the invention. These types of
multi-channel mixers can be used in WDM, DWDM and TDM optical
networks or other types of optical networks.
[0094] In FIG. 8A a multi-channel mixer 200 is used to dynamically
reconfigure N converted output frequencies. In this case light in
the form of long wavelength beams at N signal frequencies
.omega..sub.s1 through .omega..sub.sN impressed with information is
input into multi-channel mixer 200. Then, a light beam at an
appropriate pump frequency .omega..sub.p is selected for performing
DFG. Specifically, pump frequency .omega..sub.p can be selected in
any one of the multiple short wavelength channels for which
multi-channel mixer 200 has been designed in accordance with the
invention. The pump frequency .omega..sub.p determines, through
DFG, the frequency of output beams at output frequencies
.omega..sub.out1 through .omega..sub.outN (based on
.omega..sub.out=.omega..sub.p-.omega..sub.s). Thus, information
input at signal frequencies .omega..sub.s1 through .omega..sub.sN
exits multi-channel mixer 200 at output frequencies
.omega..sub.out1 through .omega..sub.outN. The N converted output
frequencies .omega..sub.out1 through .omega..sub.outN can
correspond, e.g., to WDM channels of an optical network.
[0095] FIG. 8B shows a multi-channel mixer 202 used for frequency
broadcasting also referred to as wavelength broadcasting. In this
case light at each of N signal frequencies .omega..sub.s1 through
.omega..sub.sN is converted into M output frequencies by using M
pump frequencies .omega..sub.p1 through .omega..sub.pM. Once again,
the conversion is accomplished by DFG.
[0096] FIG. 8C shows a multi-channel mixer 204 used for
reconfigurably dropping frequencies or wavelengths. This is
performed on N signal frequencies .omega..sub.s1 through
.omega..sub.sN by converting them using L pump frequencies
.omega..sub.p1 through .omega..sub.pL to output frequencies outside
the range of frequencies supported by the WDM network. By doing
this, selected signal frequencies can be dropped from the WDM
network. Once again, this operation can be performed by DFG.
[0097] FIG. 8D shows a multi-channel mixer 206 used for switching
or guiding N signal frequencies .omega..sub.s1 through
.omega..sub.sN with the aid of a reconfigurable pump frequency
.omega..sub.p. It will be clear to a person skilled in the art that
channel drop, switch, sample as well as many other useful functions
can be realized using multi-channel mixers 200, 202, 204 and 206 in
WDM networks. In fact, multi-channel mixers 200, 202, 204 and 206
can be configured for phasematching wavelengths whose location and
spacing is defined by the International Telecommunication Union
(ITU) standards. Furthermore, multi-channel mixers of the invention
employed in networks can use any suitable frequency mixing
operation to perform the required functions. A person skilled in
the art will realize that the functions of the various light beams
will be chosen by the designer. Depending on the frequency mixing
operation, pump beams, signal beams, low-power beams, high-power
beams, continuous-wave beams, pulsed beams as they are known in the
art, can all be appropriately manipulated by multi-channel mixers
according to the invention and any of these beams (with the
exception of continuous-wave beams) be impressed with
information.
[0098] FIG. 9 shows a multi-channel mixer 210 with a polarization
control system 212 for rendering mixer 210 polarization insensitive
or polarization diverse. Mixer 210 has a QPM grating 214 engineered
in accordance with the invention in a waveguide 215 produced in a
nonlinear optical material substrate 216. Polarization control
system 212 has a polarizing beam splitter 218 for splitting light
220 delivered from a fiber 222 into its two orthogonal
polarizations. After the split, p-polarized light 220A is coupled
into mixer 210 with the aid of coupling element 224 from the left.
Meanwhile, s-polarized light 220B follows a path defined by mirrors
226A, 226B and 226C. Along this path a coupling element 228 ensures
that s-polarized light 220B is efficiently in-coupled into mixer
210 and a half-wave plate 230 rotates s-polarized light 220B by 90
to coincide in its polarization state with p-polarized light 220A.
After being rotated, light 220B is in-coupled into mixer 210 from
the right.
[0099] Output light 238 from multi-channel mixer 210 exits to the
right and left from mixer 210. After retracing the paths of input
light 220A and 220B output light 238 passes through beam splitter
218 and back into fiber 222. The present embodiment conveniently
uses a circulator 236 for managing intput light 220 and output
light 238. Light 220 is delivered from fiber 234 via circulator 236
into fiber 222. Output light 238, traveling in the opposite
direction from light 220, enters circulator 236 and is passed on to
fiber 232.
[0100] A person skilled in the art will recognize that polarization
control system 212 can be replaced by alternative systems
performing the same function. These systems can employ several
components selected among elements such as polarization mode
separators, polarization rotators, optical isolators, optical
circulators, optical fibers, polarization maintaining fibers and
polarization controllers to achieve the same functionality as
system 212.
[0101] Finally, the performance of multi-channel mixers engineered
in accordance with the invention is illustrated in the graphs of
FIG. 10. These graphs represent a comparison of SHG wavelength
tuning curves for a single channel prior art mixer in (a), and
two-channel, three-channel and four-channel mixers in (b), (c) and
(d) respectively. The closed circles are measured results and the
solid lines are the theoretical fits. The efficiencies are relative
to the peak efficiency (.apprxeq.500%/W) of a one-channel
mixer.
[0102] A person skilled in the art will recognize that
multi-channel mixers of the invention can be further modified in
many ways to suit the particular needs at hand. Accordingly, the
scope of the invention should be determined by the following claims
and their legal equivalents.
* * * * *